1. Field of the Invention
This invention relates to trimellitic acid anhydride (TMA) and more particularly is concerned with an improved process for the preparation of high purity trimellitic anhydride from trimellitic acid (TMLA). The invention has particular applicability when the trimellitic acid has been produced by the oxidation of a 1,2,4-aliphatic-substituted benzene with molecular oxygen in the liquid phase and in the presence of a heavy metal catalyst.
2. Background
Trimellitic acid, the 1,2,4 benzene tricarboxylic acid, is useful as an intermediate in the production of quality plasticizers and polyester resins. For these applications, in which trimellitic acid is esterified with a monohydric or a polyhydric alcohol, the evolution of water as an esterification byproduct together with the attendant difficulty of eliminating water from esterification reaction mixtures favors the desirability of employing trimellitic acid as the anhydride rather than as the acid. Resins and plasticizers may further require a trimellitic anhydride which is relatively free from color bodies and also free from the heavy metals employed as catalysts for the air oxidation of aliphatic-substituted benzenes to produce trimellitic acid. A Delta E color of below 1 is often specified for trimellitic anhydride used in white or transparent resins, and a metal content of less than about 50 p.p.m. (parts per million) is desirable to achieve good color and oxidation stability. While the more commonly employed maleic and phthalic acid anhydrides are readily prepared by thermal dehydration of the corresponding acids, and the anhydrides are easily purified by atmospheric pressure sublimation, trimellitic anhydride cannot be processed in this manner. Firstly, the acid requires temperatures in excess of 200.degree. C. for thermal dehydration to take place, and even at these temperatures dehydration is not complete. Secondly, trimellitic anhydride is essentially nonvolatile and must be distilled at temperatures above 250.degree. C. under vacuums on the order of 10-60 mm mercury absolute to prevent color degradation. Also, to increase the ordinarily-slow rate of dehydration, it has previously been proposed to employ chemical dehydrating agents such as acetic anhydride, sulfuric acid, phosphorus pentoxide, or the like to dehydrate the last traces of trimellitic acid before distilling the anhydride. These chemical dehydrating agents are costly to recover and regenerate and consequently impose an expensive operating burden on existing processes for the purification of trimellitic anhydride. Furthermore, their use in some cases results in the substitution of one impurity for another.
Accordingly, an object of the present invention is to provide an improved process for preparing high purity trimellitic anhydride from trimellitic acid having a Delta E color below 1.0 and a metal content below 50 p.p.m. Other and more particular objects will become apparent as the description of this invention is set forth in detail hereinafter.
Suitable silicon compounds include silica compounds activated in the purification process of trimellitic anhydride. Representative silicon compounds of this type include colloidal silica such as ammonia stabilized silicon sol in water and stabilized silica sol in hydrocarbon solvent. Silicon compounds which are not activated per se in the purification of trimellitic anhydride are suitably activated and are advantageously utilized in my novel process. Silicic acid is representative silicon compound which has to be activated. Silicon compounds of this type are activated by organic acids and anhydrides. Suitable organic acids and anhydrides are derived from aliphatic hydrocarbons. Preferred anhydrides have about 2 to 8 carbons atoms while preferred acids have 1 to 8 carbon atoms. Suitable acids and anhydrides include acetic acid, propionic acid, propionic anhydride, formic acid, maleic anhydride and 2-ethyl hexanoic acid. Acetic acid anhydride is the preferred activation agent for silicon compounds.